bug#58158: 29.0.50; [overlay] Interval tree iteration considered harmful

[Eli Zaretskii <eliz@gnu.org>]

> From: Richard Stallman <rms@gnu.org>
> Cc: eliz@gnu.org, 58158@debbugs.gnu.org, monnier@iro.umontreal.ca
> Date: Fri, 30 Sep 2022 21:57:36 -0400
> 
>   > > What has to do with overlays.  To name a few: overlay-at, overlays-in,
>   > > next-overlay-change, previous-overlay-change, overlay-lists, ...
> 
> Those operations can be done in C with no risk of signaling an error.

I think such assumptions were proven dangerous at best in the long
run.  The way Emacs develops, we constantly add more and more hooks to
C code, and more and more direct calls into Lisp from C.  These
invalidate any assumptions about "no risk of signaling an error", even
if they were originally true when the code was first written.

Moreover, we test for QUIT in operations that can be prolonged ones
(and for a good reason), so any long loop in C could potentially throw
to top level if the user pressed C-g.

All in all, experience shows that making such assumptions in Emacs is
unsafe.